So my two queries which I wold normally just union, I have to deal with as follows:
1. Create a variable for the merged dimension.

2. Create a variable for the query 1 Project Manager as a detail of the merged dimension - call it "No1-PM"
3. Create a variable for the query 2 Project Manager as a detail of the merged dimension - call it "No2-PM"
4. Create a new variable on a formula if No1-PM is NULL then query 1 PM ELSE No1PM - call it "The Project Manager"

Repeat steps 2-4 for each field I want in my report.

(It's complex, and this description may not be 100% accurate, but you get the idea)
I just know that if my Union Query approach wored, this would be vastly simpler!

What makes it really difficult is that I'm never sure about whether the underlying universe's / SQL stuff is right. In this organisation those roles are seperated out.

So I have no access to detailed error logs, etc, to try understand why it doesn't work.

If you knew all that, what made you think you had to declare an alias .
You don't.

Does the generated BO union code run in a SQL tool?

Does the proposed union contain the same amount of columns with the correct data types, on both sides of the query.
Sometimes you can create dummy universe objects that return blanks to help create Unions In BO._________________Next Available for Contract Work - January 2018

From there, you'll see an icon that looks like a piece of paper. That's the view script button and will show you the generated SQL._________________Current version I'm using: 4.1 SP3 over Oracle. Well versed in SQL Server too, including SSIS

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Prior versions used: BO3,4,5,6.5,XIr2,XI3, 4.1 - yes, I have been using BO since 1996!